In FIGS. 1 and 3 a known insulated gate bipolar transistor (IGBT) 10 is shown, which includes an active cell with layers of different conductivity types in the following order between an emitter electrode 2 on an emitter side 22 and a collector electrode 25 on a collector side 27 opposite to the emitter side 22: an (n+) doped source region 3, a (p) doped base layer 4, which contacts the emitter electrode 25 in a contact area 24, an (n−) doped drift layer 5, an (n+) doped buffer layer 52 and a (p) doped collector layer 6. A gate electrode is arranged on the emitter side 22. In FIGS. 1 to 4 an IGBT with planar gate electrode 7 is shown, whereas in FIG. 5 a known IGBT is shown, which has a trench gate electrode 75.
FIGS. 2, 4 and 5 show another known IGBT, in which an (n) doped enhancement layer 8′ is arranged between the base layer 4 and the drift layer 5. The enhancement layer 8′ has a higher doping concentration than the drift layer 5.
Known enhancement layer doping concentrations have been limited to 1*1016 cm−3 in order to prevent excessive electric fields and therefore degradation of the blocking performance.
As the carrier concentration near the active cell is enhanced by such an enhancement layer 8′, such IGBTs with an enhancement layer 8′ can be superior compared to known IGBTs having no enhancement layer in view of higher safe operating area (SOA) and low on-state losses.
FIG. 3 shows electrical properties and effects for a known IGBT without enhancement layer, and FIG. 4 such effects for a known device having an enhancement layer 8′. It is shown how an n-type enhancement layer 8′ can improve the carrier spreading from the cell by creating a barrier and reducing the amount of holes that can reach the cathode (PNP hole drainage effect). This can improve the PIN effect, increase the plasma concentration and lower the on-state losses.
However, the electric field at the n-enhancement/p-base junction 8′, 4 also increases. Practical enhancement layer doping concentrations have therefore been limited to 1*1016 cm−3 to prevent excessive electric fields and therefore degradation of the blocking performance.
U.S. Pat. No. 6,147,381 A shows a known IGBT having a planar gate, which includes a (p) doped base layer, an (n+) doped enhancement layer below the base layer, as well as (n+) doped layers on both sides of the base layer, thus completely surrounding the base layer. A floating (p) layer is arranged below the enhancement layer. The (p) floating layer is heavily doped and completely covers the area below the contact area and extends laterally far beyond the contact area.
The floating layer forms a main blocking junction which shields the (p) base junction from high fields; i.e., to inhibit the course of the equipotential lines from reaching as far as the lower edge of the base layer. However, due to the main blocking junction, the charge has a restricted access from the channel in terms of spreading.
US 2008/258208 A1 shows an IGBT having a rather complex structure, in which field plates at source potential are arranged below a trench gate. (P) doped layers are arranged as bubbles below the rounded trench gate bottom. Within such a highly doped (p) bubble a small highly doped (n) bubble is arranged. The bubbles can be used to improve blocking, or to shield the field because the trench gate rounding results in high peak fields.
As the trench electrode/field plates are terminated within the enhancement layer, the electric field will be even higher due to the higher doping of the enhancement layer compared to an arrangement in which the trench gate is terminated within the drift layer. The (p) bubble is used in the device known from US 2008/258208 A1 to achieve the blocking. Due to the presence of the (p) bubbles, the highly doped enhancement layer can extend further in a direction of the drift layer for having lower on resistance.